By Atticus discussion of the "mob"threat, what do we learn about Atticus character?

Atticus reveals his fatal flaw: he tends to be overly optimistic or unrealistically hopeful at times. For instance, his prediction that the Ku Klux Klan will never return is mistaken, and even though he doesn't believe anyone will cause trouble at the jail on a Sunday night, the town members prove him wrong. It is also noticeable unclear as to whether Atticus knew Mr. Underwood was looking out for him until after the mob disperses. If he did not know, then he could have put himself in considerable danger.